Instrument transformers for installation on high voltage power lines, which may include those transformers used for protective relaying and metering, are large and expensive. This is especially true at higher power line voltages. For instance, instrument transformers for installation on 230 kV lines may cost more than $100,000 US each. Replacement of instrument transformers is thus very costly in terms of capital costs. It is also very costly to replace instrument transformers due to the necessity to power down the power line while doing so. The large size of the instrument transformers also means that installation/removal and transportation costs are high.
It is quite common in legacy installations (such as at a substation) that the only instrument transformers that are installed are those used for protective relaying. These instrument transformers are typically designed to operate during large fault currents or voltages and are therefore not optimized for accuracy at normal currents and voltages. For example a relaying current transformer may have a large magnetic core and high core losses.
When instrument transformers optimized for metering applications are provided in an installation, they may be subject to degradation in accuracy over time. This may be due to magnetization from surge voltages or currents, insulation breakdown, degradation due to environmental stresses, etc.
It is therefore common in legacy installations to have inaccuracies in the measurement of voltage, current and therefore power flow due to the degradation and/or inherent inaccuracy of the installed instrumentation.